Viscometer

ABSTRACT

A viscometer in which the angular position of an element responsive to the viscosity of a liquid, the viscosity of which is to be monitored, is sensed by a magnetic resistive sensor adapted to supply intermittent electrical signals to an electronic means recording the angular position.

This invention relates to viscometers, and in particular to an improvedsensing arrangement and control arrangement therefor.

The measurement of viscosity is important in many production processes,and there have been a large number of proposals for viscosity measuringapparatus.

One previous proposal is that of my own earlier United Kingdom patentapplication No. 2 058 341, which used a continuously rotated,resiliently mounted bob immersed in the test liquid; the change inangular position of a detection arm connected to the bob with change inliquid viscosity was recorded once each bob revolution by detectormeans, conveniently optical detector means.

An alternative arrangement is that disclosed in U.S. Pat. No. 4,448,061in which a transducer is provided with a stator included in the drive ofa resiliently yieldable connection subject to a constant speed drive,and having a rotor which turns relative to the stator as the connectionyields or recovers in response to viscosity changes; the transducerprovides a continuous signal, the strength of which changes withviscosity changes, and which is continuously delivered to a digitaldisplay or readout which is responsive thereto.

It is an object of my invention to provide a viscometer having anintermittent signal delivered to a recorder e.g. a visual display, butwith an accuracy at least equal to that of known apparatus withcontinuously delivered signals.

Thus according to one feature of my invention I provide a viscometer inwhich the angular position of an element responsive to the viscosity ofa test liquid is sensed by a magnetoresistive sensor. Preferably thesensor indicates the relative angular positions of two rotatableelements, one of which is responsive to the liquid viscosity. Thus Iprovide a viscometer for use in monitoring the viscosity of a liquid,which includes a rotatable support element, drive means to rotate saidsupport element, a rotatable driven element coaxial with said supportelement and having a part which is to be immersed in a liquid theviscosity of which is to be monitored and which is to be subjected to aviscous drag which changes as the viscosity of the liquid changes,resilient means connecting the elements and which yields with variationin the relative angular position of the elements as the viscous dragincreases, electrical means to sense the variation in the relativeangular position of the elements and to provide an electrical outputsignal of a magnitude dependent upon said relative angular position, andelectronic means communicating with the electrical means and arranged torecord the variation in angular position characterised in that theelectrical means is a magnetoresistive sensor mounted to rotate with oneof the elements and responsive to magnetic flux producing means mountedto rotate with the other of the elements, in that the electronic meansis connected to the electrical means by an analogue to digitalconverter, and in that the electronic means is a microprocessor systemadapted to derive intermittent electronic signals from the electricalmeans by intermittent sampling of the output of the analogue to digitalconverter.

In order that the viscometer of the present invention can provide rapidupdates of any change in viscosity, I arrange my magnetoresistive sensorto provide several indicator signals each second, usually at least 10signals per second and conveniently 20 signals per second.

As is well known, the electrical resistance of thin ferromagnetic,layers depends on the angle between the direction of the electricalcurrent flowing in the layers and the direction of the imposedmagnetisation (the magnetoresistive effect). For my ferromagnetic layerI prefer an alloy of 80% nickel and 20% iron, since such an alloy has asignificant resistance charge of between 2% and 3%, whilst having stableand repeatable characteristics over a wide temperature range; though inalternative embodiments I may use an alloy having between 70% and 90%nickel. Thus such a material is particularly useful as a sensor in myviscometer when used to record the viscosity of a liquid.

Nevertheless, though the magnetoresistive characteristics are stable andrepeatable over wide temperature range, there is a square-lawrelationship. This can be corrected for, to provide a linearcharacteristic with temperature, as by use of an auxiliary magneticfield; or as by special geometric structures such as the use of a highconductivity metal e.g. gold, deposited at an angle of 45 degrees to theX or current flow axis such that an external magnetic field by the Ydirection can rotate the resultant magnetisation towards the Y axis soaltering the electrical resistance of the sensor.

These correction techniques, though usable in conjunction with this onefeature of my invention, nevertheless introduce some complication intothe apparatus, so that in accordance with a further feature of myinvention I provide a viscometer which includes sensor means adapted toproduce an electrical output dependent upon liquid viscosity, andcomputer means to which the electrical output can be fed, the computermeans having an output to a viscosity recorder and having at least oneinput for correction means, whereby the viscosity recorder will providea viscosity value corrected for at least one parameter. Though amagnetoresistive sensor has an output which over a significant angularrange is nearly linear, for many applications a true linear output withangle is required so that one parameter will be the linearisation ofthis sensor characteristic to output, i.e. to provide a linear relationbetween spindle relative angular deflection and output; but additionallyor alternatively the parameter may be one or more of temperature,spindle type, speed of rotation and the units in which the viscosity isto be recorded, as by visual display or by printed form. By providingmeans to program the computer means so as to correct the instrumentoutput signal it is possible to achieve an accuracy of plus or minus0.5% of full scale, whilst otherwise using a standard instrument, ormore cheaply a "standard" instrument less manual correctors. Use ofcomputer means speeds any necessary corrections and avoids operatorerrors unavoidable with repetitive human calculations.

The instrument may be in a single housing, or as separate modules e.g.sensor, display, printer etc.

Depending upon the application, a particular instrument could berequired to measure viscosity using a bob rotational speed in the rangefrom 0.3 rpm to 300 rpm. If neither auto-rangeing nor portability arerequired, the bob may conveniently be driven by a synchronous AC motor,with a gearbox to permit manually selected ranges for determining thebob rotational speed; for applications in which the instrument needs toauto-range or needs to be portable, then a DC motor will conveniently beused with in the auto-range model the gearbox stepping down the bobrotational speed and allowing major speed steps to be set manually, andwith in the portable embodiments the gearbox providing rangechange stepsas for the AC motor driven embodiments. For portability, a DC motorenergised by 6 V or 12 V batteries may be adopted.

The invention will be further described by way of example with referenceto the accompanying drawings, in which

FIG. 1 is a schematic side elevation of a viscometer according to theinvention;

FIG. 2 is block diagram of a circuit to convert and correct electricaloutput signals from the viscometer of FIG. 1 to a viscosity value;

FIG. 3 is a circuit diagram for the magnetoresistive sensor.

The liquid 2 under test is in container 4, preferably at a specifiedtest temperature or at a known constant temperature, but this is notessential since a rapid-response temperature probe 5 can be introducedinto container 4 to monitor liquid temperature; whilst in thisembodiment the container 4 has a specified volume of test liquid 2, inan alternative embodiment container 4 may be part of a liquid conduit,so that the liquid under test is continually replaced.

Also positioned in container 4 is a rotatable bob 6, which may be of anyconventional form; in FIG. 1 the bob is shown as a disc, but in anotherembodiment may for instance comprise two concentric open-endedcylindrical elements, one within the other, one a driver and the otherbeing driven by the viscous drag of the test liquid between theelements. The bob, disc and spindle may be of any convenient shape orsize.

Bob 6 is suspended by lower arm 8 on upper arm 10, upper arm 10 havingtwo needle ends and being rotatably located in needle bearings 12 and14, in support 16. Support 16 can be rotated by motor 18 by way of gearbox 20 and drive shaft 22; between support 16 and upper arm 10 is aspiral spring 24 which rotates upper arm 10 and thus bob 6, the bob 6 inthis embodiment having a permitted angular displacement of up to 120degrees relative to support 16. Means can be provided to reduce therotational speed of support 16 should the relative angular displacementof bob 6 appear likely to exceed the specified relative angulardisplacement, in this embodiment by way of variable speed motor e.g.D.C., Motor 18; such means can alternatively be used to increase therotational speed of support 16 should the relative angular displacementbe too small to use fully the available range of the apparatus, that isto provide automatic rangeing in response to microprocessor signals asdescribed below, or if preferred manual rangeing. Additionally thismeans can be used to maintain a constant shear stress in liquid 2. Motor18, gear box 20 and support 16 are located within a housing 34, gear box20 including ratio-change means 26 permitting the gearing ratio betweenthe motor 18 and support 16 to be altered.

Carried by upper arm 10 are a pair of magnets 28, between which andcarried by support 16 is a magnetoresistive sensor 30. Relative angularrotational between upper arm 10 and support 16 alters the resistance ofmagnetoresistive sensor 30; input and output electrical lines 32 extendthrough housing 34 to brushes 36 engaging slip rings 38, which areelectrically connected to the magnetoresistive sensor 30. As seen inFIG. 2, the other ends of electrical lines 32 are connected to theanalogue to digital converter 40 and thus to microprocessor 42.

Also connected to the analogue to digital converter 40 is the electricalline 41 from the temperature probe 5, and the line 43 etc from otherprobes (not shown) such as one measuring that instrument temperaturelikely to affect the amplitude of the signal along lines 32. Alsoconnected to the microprocessor 42 are input lines 44 applying steppedor continuously variable corrections, for instance for the spindle typeand the conversion factor for the units in which the viscosity is to berecorded, e.g. displayed as by visual display 46, which in a preferredembodiment is part of the instrument; or printed by a unit connected toline 48. Preferably the viscosity measurements are made at apredetermined fixed temperature to permit ready comparison, but as anadditional feature if the viscosity-temperature characteristics for thetest liquid is already known this can be added to the material programmemory 50 as an adjunct to program memory 52 and referred toautomatically by the microprocessor for the appropriate correctionfactor in accordance with the input signal from probe 5.

Preferably microprocessor 42 is part of the instrument with an integralvisual display unit 46 capable of both numeric and alphabetic characterscorrectly formatted by the microprocessor; though in an alternativeembodiment the visual display unit can be remotely positioned at a moreconvenient location. The microprocessor 42 permits rapid yet discretesampling, generally 10 samplings a second or above and in one embodimentat 20 samplings each second, providing speedy updating of viscositymeasurement. If preferred the microprocessor can be instructed toaverage the readings over a given time period so that for instance thevisual display does not "jitter", as a selected alternative to lessfrequent sampling. The fast updating can be used to control by way ofoutput line 54 the speed of motor 18 in order to obtain automaticrangeing as mentioned above. If required the microprocessor can beinstructed to display only the maximum or minimum value of the viscosityrecorded during the period.

The microprocessor also permits auto-zeroing, to allow the instrument tozero itself when required. This is achieved by allowing bob 6 to rotatein air, with the microprocessor input set at "zero", so that thecomputer reduces any offset reading to zero using a recognised programin memory 52.

It will thus be appreciated that the viscometer of this inventionpermits non-linearities and temperature effects to be corrected "inside"the instrument by using a suitably programmed microprocessor and memory.The output can be displayed and/or printed in units of dynamic viscositye.g. mPaS or in units of kinematic viscosity e.g. cSt, by taking accountof the liquid specific gravity at the temperature as recorded by probe5. Nevertheless means for manual instrument adjustment, such a sensoradjustment 55 or for correcting or trimming variations in the rate ofspring 24 to compensate for manufacturing tolerances can be provided inaddition to the correction allowed by the microprocessor.

It will also be appreciated that my viscometer is a torque meter, andthat it can be so used.

Whilst most applications of my viscometer will make use of its highaccuracy potential of better than plus or minus 0.5% of full scale, andof its frequent sampling in order to monitor rapidly any changes in theviscosity of liquid 2, it will also be appreciated that this viscometercan be used as an automatic control instrument for processes requiring aconstant liquid viscosity or specific gravity achievable by alteringassociated process parameters or apparatus operation by way of outputline 54.

Usefully the sensor will be protected from stray magnetic fields by ashield such as annular shield 60.

It will be understood that the instrument can be of unitary constructionwithin a single housing or be "unbundled" into discrete units, perhapswidely separated as when process units are controlled or monitored froma central point. Usefully however the components shown in FIGS. 1 and 2will together form a one-piece or "bundled" instrument. Whilst theinstrument will usually have a single chip microprocessor with on-boardRAM (random access memory) and I/O (input/output) circuitry such as thatleading to I/O line 62, in an alternative embodiment a so-called singlechip microcomputer with on-board RAM, ROM (read only memory) and I/Ocircuitry could be used.

I claim:
 1. A viscometer for use in monitoring the viscosity of aliquid, which includes a rotatable support element, drive means torotate said support element, a rotatable driven element coaxial withsaid support element and having a part which is to be immersed in aliquid the viscosity of which is to be monitored and which is to besubjected to a viscous drag which changes as the viscosity of the liquidchanges, resilient means connecting the elements and which yields withvariation in the relative angular position of the elements as theviscous drag increases, electrical means to sense the variation in therelative angular position of the elements and to provide an electricaloutput signal of a magnitude dependent upon said relative angularposition, and electronic means communicating with the electrical meansand arranged to record the variation in angular position characterisedin that the electrical means is a magnetoresistive sensor mounted torotate with one of the elements and responsive to magnetic fluxproducing means mounted to rotate with the other of the elements, inthat the electronic means is connected to the electrical means by ananalogue to digital converter, and in that the electronic means is amicroprocessor system adapted to derive intermittent electronic signalsfrom the electrical means by intermittent sampling of the output of theanalogue to digital converter.
 2. A viscometer according to claim 1 inwhich the electronic means is adapted to derive at least 10 signals eachsecond.
 3. A viscometer according to claim 1 in which themagnetoresistive sensor includes a ferromagnetic layer of an alloyhaving between 70% and 90% nickel, preferably 80% nickel and 20% iron.4. A viscometer according to claim 1 characterised in that theelectronic means includes computer means to which the electrical signalsare supplied, the computer means having an output to a viscosityrecorder and having at least one input for correction means so that theviscosity recorder can provide a viscosity value corrected for at leastone parameter.
 5. A viscometer according to claim 4 characterised inthat the parameter is the variation in angular position so that theoutput to the viscosity recorder provides a viscosity valuerepresentative of a linear relation between variation in angularposition and signal amplitude.
 6. A viscometer according to claim 4characterised in that the viscosity recorder is a visual display unitand printer combination.
 7. A viscometer according to claim 1characterised in that the drive means can rotate said support element ata speed in the range 0.3 rpm to 300 rpm.
 8. A viscometer according toclaim 1 characterised in that the drive means is a synchronous ACelectric motor connected to a manually-charged gearbox.
 9. A viscometeraccording to claim 1 characterised in that the drive means is a DCelectric motor connected to a gearbox adapted to provide an auto-rangefacility.
 10. A viscometer according to claim 1 characterised in thatthe relative angular position is up to 120°.